Add documentation for init command (#2125)

COMMANDS.md

@@ -21,6 +21,7 @@ The most common subcommands are:
 * [`pyenv prefix`](#pyenv-prefix)
 * [`pyenv hooks`](#pyenv-hooks)
 * [`pyenv shims`](#pyenv-shims)
+* [`pyenv init`](#pyenv-init)
 * [`pyenv completions`](#pyenv-completions)
 
 ## `pyenv commands`
@@ -347,8 +348,19 @@ List existing pyenv shims.
     /home/user/.pyenv/shims/python3-config
     /home/user/.pyenv/shims/python-config
 
+## `pyenv init`
+
+Configure the shell environment for pyenv
+
+    Usage: eval "$(pyenv init [-|--path] [--no-rehash] [<shell>])"
+
+      -                    Initialize shims directory, print PYENV_SHELL variable, completions path
+                           and shell function
+      --path               Print shims path
+      --no-rehash          Add no rehash command to output     
+
 ## `pyenv completions`
 
 Lists available completions for a given pyenv command.
 
-    Usage: pyenv completions <command> [arg1 arg2...]
+    Usage: pyenv completions <command> [arg1 arg2...]